Objective: To spatially co-register and to flux calibrate several
instruments based on observations of a star.

Scientific Case:
It is crucial to monitor instrument sensitivity changes during the SOHO mission.
It is also important to place intensity measurements of various instruments on a
common scale and to spatially co-register them if we are to combine observations with the different
instruments. Both purposes can be served by observing stars.
Several UV-bright stars pass close enough to the sun during the course of the
year that they can be observed by SUMER, LASCO and UVCS. They should
be observed each year during the mission to co-register the instrument
pointings, to track sensitivity changes, and to compare with IUE,
Voyager and optical intensity calibrations. None of these stars is
known to vary, so strict simultaneity may not be necessary, but it is
clearly desirable. Unfortunately, none of
these stars are likely to be detectable below the Lyman limit, so
these calibrations are restricted to the UVCS first order UV ranges
and the White Light Channel, the longer wavelength part of the SUMER
range, and LASCO. Line ratios must be used to transfer the calibrations to
shorter wavelengths.

OBSERVABLES

UVCS: Intensities over the full wavelength ranges in the UV channels
and count rate in the WLC

UVCS will repeatedly place the UV channel slits and the WLC focal plane aperture to let the star
drift across them. For 11" spatial resolution, the star drifts past in 268/sin
seconds (where is the angle between the slit and the apparent path of the star;
for the example below).
Repeated drifts across the slit will build up the exposure time,
and they will also sample different parts of the detector and
different internal occulter positions. The detector masks will
cover the entire spectral ranges
of the UV detectors at full spatial and spectral resolution. In order to meet
telemetry limitations with several readouts during the drift across the slit
(1-2 minute), only about 19 (TBC) pixels in the spatial direction will be read out.
It will be necessary to select different grating positions for the different
crossings to cover the full wavelength ranges. The direct O VI and Redundant Ly contributions fall
on top of each other in the O VI channel. This is not a problem for the sparse
emission line spectrum of the corona, but it is a problem for a stellar continuum. Different
grating positions will place the deep stellar Lyman absorption features
at different positions.

Details of the mirror motions and slit width depend upon the target star and the distance from
Sun center. As an example, we consider the approach of Sco to
the east limb on Nov. 26. We conservatively take a dwell time much larger than the
crossing time. As we gain experience with the satellite, more efficient
choices can be made. A single drift across the aperture should provide about
430 counts per pixel at 1240 and 50 counts per pixel at 1040 (with
about 93 counts per pixel superposed from the Redundant Ly contribution) when the
star is at 2.37 (assuming the spectrum to be 2 pixels high). The internal
occulter will reduce these values to 155 and 21 counts per pixel, respectively, at
1.5 . This sequence includes 14 drifts across the slit, one of which
crosses the WLC.